Conventional gas turbine engines, typically have a turbine section for extracting work from working medium gases. The turbine section generally includes airfoils, such as rotating blades and stationary vanes. It is industry practice to protect turbine airfoils from oxygen, sulfur, and high temperatures, which the airfoils encounter during normal engine operation, using coatings. Since coatings preserve the integrity of the airfoils, coatings improve the service life of airfoils, and allow the airfoils to operate at optimum temperatures.
Packed, plasma spray, and physical vapor deposition are conventional processes for applying coatings to airfoils. During these coating processes, portions of the airfoils require masking or shielding from the coating. For example, a trailing edge of some airfoils requires masking from ceramic coatings. Ceramic coatings will insulate the trailing edge from cooling air, and potentially cause trailing edge distress in service. Trailing edge distress includes a variety of phenomenon, such as oxidation, erosion and cracking, which lead to performance losses while the blade is in service. If the distress is substantial, the blade will not be repairable during overhaul, and the blade is not returned to service.
Several possible methods for masking the trailing edge of airfoils during coating have been sought. One possible method for masking the trailing edge of a part requires applying a metallic tape to the area where the coating is undesirable. Due to the high temperatures employed during coating, the tape tends to burn away unpredictably. Since the tape bums late in the process, the tape only initially protects the vulnerable areas, but using tape is not the optimum solution for several other reasons. First, because the tape bums unpredictably, tape is unsuitable in a production environment where consistent results are imperative. Second, the use of tape is effective only for certain coatings and certain application processes. For example, when using the physical vapor deposition process, tape cannot be used, because the tape and the coating can interact on the molecular level during the process. This interaction can prevent the coated airfoil from having the required properties.
Another possible method for masking the airfoils uses a fixture. The fixture for use with a vane includes two enclosures, a masking member, a supporting slat, and a connecting member. Each enclosure surrounds and protects portions of the vanes from coating. The supporting slat attaches the enclosures to one another. The connecting member fixes the masking member to the supporting slat. The masking member is typically in a fixed position spaced from the trailing edge of the airfoil, thus defining a gap between the masking member and one surface of the airfoil. Since there is a gap between the masking member and the surface of the airfoil, coating particles can enter this gap and adhere to the trailing edge during coating. One solution that temporarily decreases coating on the trailing edge is bending the connecting member. This decreases the gap, which consequently decreases the amount of coating on the trailing edge. However, this solution is temporary, and coating begins to build up due to movement of the masking member resulting from vibrations caused by the coating process and/or the thermal growth experienced by the fixture as its temperature increases during coating.
When the fixture undergoes vibration and/or thermal changes, the masking member can come into repeated contact with the vane, which can cause removal of existing coating or wear of the vane. Removal of existing coating exposes the vane, during use, to the harsh engine environment, and leads to the airfoil being damaged in service. Wear of the vane itself can create crack initiation sites that might grow in service. Both removal of existing coating or wear of the vane are unacceptable, because they can reduce the engine performance and vane service life.
In addition, when the fixture undergoes vibration and/or thermal changes, the masking member can move away from the surface of the vane, or out of alignment with the trailing edge of the vane. Such displacement of the masking member can allow the coating particles to contact and adhere to the trailing edge more easily. Sometimes grit, vapor or water blasting can remove undesired coating along the trailing edge, but such coating removal operations may add significantly to the cost of the overall coating operation. In other cases where undesired coating cannot be removed, the coating can cause trailing edge distress in service. Trailing edge distress can lead to performance losses and decreases the service life of the airfoil, both of which result in significant costs.
Furthermore, using the prior art fixture can result in the masking member and the airfoil joining due to coating spanning these two parts and drying. This joining, known as bridging, is undesirable because separation of the masking member from the airfoil under such conditions can cause damage to the coating, as well as the airfoil. Damage to either can lead to performance losses and decreases in the service life of the airfoil.
Therefore, an improved fixture is sought, which effectively masks a portion of the midspan region of an airfoil during coating, which does not wear away existing coating or create crack initiation sights, which is inexpensive to manufacture, which is suitable for a production environment, and which minimizes bridging.